The present invention relates to a technique for generating a laser beam in an ultraviolet region with a small apparatus.
To generate a laser beam in an ultraviolet region, for example, having a wavelength of about 200 nm (nanometer), a related art laser beam generating apparatus has been configured to:
(1) generate as a second harmonic of a pigment laser pumped by an excimer laser oscillating at a wavelength in the order of 300 nm and oscillating near 400 nm;
(2) generate as a fourth harmonic of a solid laser such as a titanium-sapphire laser or an alexandorite laser oscillating near at 800 nm; or
(3) generate as a fifth harmonic of an Nd laser (for example, Nd:YAG laser) oscillating at a wavelength of about 1 μm.
Each of the above-described related apparatuses, however, has the following problems in terms of maintenance, efficiency, size, and the like.
In the apparatus described in (1), a gas used for the excimer laser has problems with poison and complicated exchange and the pigment laser has a problem with degraded maintenance. This apparatus, therefore, has a difficulty in industrial applicability.
In the apparatus described in (2), since the solid state laser is used, the maintenance is improved; however, a high output visible light source is required as a pumping source for the titanium-sapphire laser. This apparatus, therefore, has problems that the entire efficiency is low, and that the entire apparatus is enlarged so much.
The apparatus described in (3) requires the generation of a higher harmonic, and therefore, has problems that the final efficiency becomes low even if the efficiency of each stage is made high, and that it is difficult to generate continuous wave of the higher harmonic.
By the way, there has been reported a technique of obtaining a laser beam having a short wavelength by wavelength conversion with non critical phase matching (NCPM) of a BBO (barium borate) crystal. This technique has an advantage that there is no degradation of a rectangular ratio (aspect ratio) of a beam due to a walk off (which is an angle between a wave number and a pointing vector and is an index in the case where an angular difference is present between the normal line of the wave plane and the energy flow; however has a disadvantage in wavelength conversion efficiency. To be more specific, in this case, an effective nonlinear constant d31 is only 1.8% of an effective nonlinear optical constant d22, and since the output is nearly proportional to the square of the effective nonlinear optical constant d31, the wavelength conversion efficiency is low. In particular, in the case of performing wavelength conversion for continuous waves, if the effective nonlinear optical constant is low, there occurs more serious problem because the wavelength conversion efficiency becomes significantly low even if an external resonator is used. Also, in the case of growing the BBO by a flux method, the growth rate is very low, failing to obtain a large-sized crystal having a homogeneous quality, with a result it is difficult to obtain a crystal having a long size. For such a crystal having a short size, an interaction length cannot be made longer, so that the BBO crystal obtained by the flux method is not advantageous in efficiency.